This study will attempt to further characterize the inhibition to myocardial perfusion that results from mechanical compression of the coronary vessels by the beating heart. Special emphasis will be placed on determining the effect of tone in the coronary vessels on this process. Experiments are also proposed which will yield a better estimate of the magnitude of the compression in the heart wall. The mechanical compression afforded the cornoary arteries during fibrillation, as is used during cardiopulmonary bypass, will be studied in detail to both measure the compression during fibrillation and identify factors which may alter it. The study will also be extended to the effect of myocardial edema on this compression both that produced by ischemia and by hypotonic agents. The study will also examine blood flow distribution in the whole heart by a new method of microsphere autoradiography. Eventual necrosis following embolization of a coronary artery will be compared to the initial patterns of ischemia (microsphere measurement) to see what correlations exist. These experiments will better define the nature of the salvageable tissue following coronary occlusion.